1. Field of the Invention
The present invention relates to a shadow mask type cathode ray tube, which is used for a television receiver, a computer display, and the like.
2. Description of the Related Art
FIG. 4 is a cross-sectional view showing one example of a conventional color cathode ray tube. The color cathode ray tube 41 shown in FIG. 4 includes a substantially rectangular-shaped face panel 42 having a phosphor screen formed on its inner face, a funnel 43 connected to the rear side of the face panel 42, an electron gun 44 contained in a neck portion 43a of the funnel 43, a shadow mask 46 facing the phosphor screen 42a inside the face panel 42, and a mask frame 47 for fixing the shadow mask 46. Furthermore, in order to deflect and scan electron beams, a deflection yoke 45 is provided on the outer periphery of the funnel 43.
The shadow mask 46 plays a role of selecting colors with respect to three electron beams emitted from the electron gun 44. xe2x80x9cAxe2x80x9d shows a track of the electron beams. The shadow mask is a flat plate in which a number of apertures, through which electron beams pass, are formed by etching.
In a color cathode ray tube, due to the thermal expansion caused by the impact of the emitted electron beams, the electron beam through aperture is shifted. Consequently, a doming phenomenon occurs. That is, the electron beams passing through the electron beam through apertures fail to hit a predetermined phosphor correctly, thus causing unevenness in colors. Therefore, a tension force to absorb the thermal expansion due to the temperature increase of the shadow mask is applied in advance, and then the shadow mask is stretched and held to the mask frame. When the shadow mask is stretched and held as mentioned above, even if the temperature of the shadow mask is raised, it is possible to reduce the amount of displacement between an aperture of the shadow mask and phosphor stripes of the phosphor screen.
However, the conventional color cathode ray tube described above suffered from the following problem. FIG. 5 is a plan view showing a part of an example of a conventional shadow mask. In a shadow mask 20, a number of apertures 21 are formed in lines. The apertures 21 neighboring in the vertical direction (vertical direction of the screen) are connected by a bridge 22. With regard to the shadow mask illustrated in FIG. 5, due to the thermal expansion of the shadow mask 20 caused by the impact of the emitted electron beams, for example, in an area B, which is a portion between the neighboring aperture lines, stress is applied in the direction indicated by arrow d. When such stress is applied, wrinkles are created in the area B, and the aperture 21 is shifted in the horizontal direction. When such a so-called local doming phenomenon occurs, electron beams do not hit the shadow mask correctly, thus causing displacement of colors, unevenness in colors, and deterioration of luminance.
Furthermore, the thickness of the shadow mask is extremely small (for example, about 0.1 mm) compared to the thickness of the mask frame to which the shadow mask is fixed. Therefore, in the initial stage of operating the color cathode ray tube, a so-called initial doming phenomenon occurs, in which only the shadow mask expands thermally. When this initial doming phenomenon occurs, the lines of the aperture are shifted in the horizontal direction, so that electron beams do not hit the shadow mask correctly, thus causing displacement of colors, unevenness in colors, and deterioration of luminance. Such problems caused by the local doming phenomenon or the initial doming phenomenon could not be prevented sufficiently even by stretching and holding the shadow mask as described above.
It is an object of the present invention to solve the conventional problems described above by providing a cathode ray tube in which a slit is formed between aperture lines, thereby preventing incorrect hitting of color electron beams caused by the local doming phenomenon and the initial doming phenomenon from occurring and thus preventing displacement of colors, unevenness in colors, and deterioration of luminance from occurring.
To achieve the above object, the cathode ray tube of the present invention is characterized by including a shadow mask made of a flat plate in which a number of aperture lines are arranged, wherein a slit extending in the line direction of the apertures is formed between the neighboring aperture lines. According to the cathode ray tube described above, it is possible to suppress the displacement of apertures due to the local doming phenomenon or the initial doming phenomenon caused by the thermal expansion of the shadow mask, and thus to prevent displacement of colors, unevenness in colors, and deterioration of luminance caused by incorrect hitting of electron beams from occurring.
In the cathode ray tube described above, it is preferable that the slit includes a slit having inclined faces opposed to each other via an opening, and the inclined faces are formed at an angle inclined such that electron beams entering the shadow mask are blocked.
Furthermore, it is preferable that the slit described above includes a slit having inclined faces opposed to each other via an opening, and the inclined faces are inclined toward the side of a vertical center line of the shadow mask by taking a rear face of the shadow mask as the starting point. According to the cathode ray tube including the shadow mask provided with the slit having inclined faces as described above, electron beams are blocked surely in the portion where the slit is formed. Therefore, while incorrect hitting of electron beams can be prevented, with regard to passing of electron beams, this shadow mask is substantially equivalent to a shadow mask in which a slit is not formed.
Moreover, it is preferable that a connected portion is further provided for linking the inclined faces opposed to each other at one portion of each inclined face. According to the cathode ray tube described above, electron beams are blocked even more surely in the portion where the slit is formed.
Furthermore, a plurality of slits described above preferably are formed separately in the line direction of the apertures. According to the shadow mask described above, while the displacement of apertures caused by the thermal expansion of the shadow mask can be suppressed, a number of bridge for linking upper and lower slits is increased, so that the hardness of the shadow mask can be secured more easily, and at the same time, the surfaces of the shadow mask located on the right and left sides of the slits are less likely to be entwined.